1. Field of the Invention
This disclosure relates generally to an automated liquid filter apparatus and method, and more particularly to a filter method and apparatus including a clamping and locking mechanism to maintain the shells of the filter in a liquid-tight closed position during a filtration cycle.
2. The Prior Art
Pressure filters of the plate and frame type have been used for many years in many different applications. Generally speaking, this type of filter may be described as a frame on which there are a number of indented loose plates with filtering surfaces, the plates being clamped together to form a series of hollow chambers and being capable of withstanding high internal pressures. The filtering surfaces are usually ribbed or grooved and covered with cloths of filtering material.
The regular plate and frame filter press consists of a filter press frame made of two end supports rigidly held together by horizontal steel bars. Upon these horizontal bars are placed a varying number of flush plates and frames clamped together, thereby forming a hollow chamber. There could be many of these plates stacked against each other and mating at the edges which are machined to form a joint surface.
This type of a filter is usually closed by a screw or hydraulic ram for pressing the plates and filter press frames together. Most of these types of plate and frame filters are normally opened and closed by hand but, in some larger size units, automatic devices are often used. The filter itself, however, in its entirety cannot be considered an automated piece of equipment because constant manual attention is required to perform various functions.
The filter press has found wider application than any other type of filter. It is structurally simple without complicated auxiliaries making for low initial cost and low installation cost. Additionally, it can be operated by unskilled labor and considerable pressures can be applied to large filter areas in a compact space. The greatest disadvantage of the filter press is its intermittent operation and the labor involved therein. Since the size of the plate and frames are generally within some reasonable size, such as 10 to 20 square feet per unit, and the thickness of the cake formation is generally up to about 2 inches, it is quite evident that if a heavy slurry were pumped to this type of a filter frequent cycling would occur. Since there are many chambers that the cake is in and since these chambers all have to be separated, the time consumed to do this is laborious and expensive. Therefore, this type of a filter is restricted on the total solids that it can handle in a given period of time. Many times it is required that the solids be discharged from the filter as a dry cake. With this type of a filter, then, it is necessary to drain the frames completely before they can be opened. If this is not done, the cake will discharge as a wet slurry. There are also many conditions where the cake is somewhat compressible or where weak structured solids are present, such that filling the frame becomes very difficult. Therefore, it becomes apparent that even though this type of filter has universal application, it also has some severe restrictions.
In order to overcome these difficulties inherent in the basic plate and frame filter, a simplified single plate type of filter was shown in U.S. Pat. Nos. 2,867,324, 2,867,325 and 2,867,326. These patents, all by the applicant, describe a simplified single plate filter that is completely automated. These filters proved to be of great value in the filtration art in that they could be placed in areas where no manual attention was required and the filters could go through the process of filtration and dry sludge removal without any manual attention.
These filters were primarily used on applications where a large volume of liquid was being pumped to the filter at a relatively moderate suspended solids concentration. Usually the total solids of the influent to the filter were less than 0.1%. However, with the large flow rates, in the neighborhood of 30 to 50 GPM/ft..sup.2, it is apparent that these filters could filter and discharge a considerable amount of solids.
One inherent characteristic of this type of filter was its limitation on operating pressures. The filter, being substantially a box that is divided in half, had to have a greater external force to keep the box closed than the internal forces that were generated by the hydraulic pressures. For instance, with 10 psi internal pressure, each square foot of the filter had to have an external counter force of at least 1,440 pounds to maintain the forces in equilibrium. Therefore, a filter with 10 square feet of filter area had to have external cylinders or external forces greater than 15,000 pounds to overcome an internal force of 10 psi. Since most of these filters were used on fairly porous suspended solids that were generated from machining operations, the pressure range in which this equipment operated was quite satisfactory.
It was found that this type of filter was extremely good for dewatering various types of sludges. Most of the dewatering jobs had higher concentrations and finer suspended solids than were ever applied to this kind of filter. A typical slurry could very well be in a 5 to 10% concentration with most of the suspended solids being finer than 300 mesh. The flow rate through these filters was reduced to a considerable extent because of the nature of the suspended solids that were being filtered. Many types of slurries actually could not be filtered effectively with this type of filter because, in order to get acceptable flow rates, higher pressures were required than could be applied to this particular type of filter. Generally, the limitation on this simplified plate and frame filter was about 10 to 12 pounds internal hydraulic pressure. Since, on certain applications greater pressure was required, the applicant developed the invention embodied in U.S. Pat. No. 3,333,693 directed to a modified, single plate, pressure filter that had the main body of the filter fixed and a seal that was the only closure and movable part of the filter. In this type of apparatus, larger filter areas could be utilized and operating pressures could be increased to approximately 20 psi. This movable seal filter, although a substantial improvement over the movable shell filter, still was not capable of high enough filtration pressures to tackle most of the different dewatering jobs.
A typical dewatering job could be defined as the filtration of coal tailings in a coal mining and processing operation where the suspended solids could be from 1 to 40% with the solids being finer than 300 mesh. These types of operations usually have some extreme fines present that also tend to lock up and close the porosity of the filter cake and reduce the flow rate considerably. On these types of sludges and many other similar ones, higher pressures in the neighborhood of 50 to 100 psi can improve flow rate and filter through-put considerably. Since the quantities of sludge also are many tons per hour, it becomes imperative that this type of a filter be capable of filtering and discharging these large quantities automatically and with the maximum through-put both in liquid and in solids.
Thus, the prior art has not provided a practical filtration method or structure to accommodate relatively high hydraulic pressures during a filtration cycle.